1. Field of the Invention
The present invention relates to a solid state image pickup apparatus and, more particularly, to a solid state image pickup apparatus having a plurality of photoelectric transducer elements each having a capacitor electrode on a control electrode region of a corresponding semiconductor transistor.
The present invention also relates to a solid state image pickup apparatus for selectively reading out a plurality of sensor signals and, more particularly, to a solid state image pickup apparatus capable of eliminating unnecessary components such as variations in dark signals and drive noise.
2. Related Background Art
A TV or SV camera with an image sensor such as a CCD or MOS sensor has an aperture mechanism. Photoelectric transducer apparatuses each having a TV or SV camera with an automatic aperture mechanism are described in Japanese Patent Disclosure (Kokai) Nos. 12759/1985 to 12765/1985.
This photoelectric transducer apparatus includes a photosensor having a plurality of sensor cells each having a capacitor formed on a control electrode of a corresponding semiconductor transistor.
In the conventional photoelectric transducer apparatus described above, noise is often mixed in an output signal read out from the photosensor cells due to variations in dark voltage generated in the cells within an arbitrary store time.
An output sign corresponding to the dark current component generated within the photosensor cell is prestored as reference optical information in an external memory in a conventional apparatus. A reference output signal derived from the reference optical information and an output signal from the actual optical information read out from the photosensor cell are compared with each other, and the output signal of the actual optical information is corrected, thereby eliminating the noise component caused by the dark voltage.
In the conventional photoelectric transducer apparatus described above, in order to constitute a photoelectric transducer system, the resultant system is undesirably complicated since a separate external circuit including a noise removal memory is required.
When a conventional photoelectric transducer apparatus is applied to, a video camera or the like, the following problem occurs. When photoelectric transducer cells are arranged in a two-dimensional matrix and scanned in the vertical and horizontal directions, holes are stored in the base of each photoelectric transducer cell in a store mode upon reception of strong light. The base potential is forward-biased with respect to the emitter potential. The potential of a vertical line connected to the emitter electrode of each photoelectric transducer cell receiving strong light is increased to cause a blooming phenomenon. In order to prevent this, it is proposed that the vertical lines are grounded for a period excluding the readout operation, thereby refreshing the charge overflowing onto the vertical line. However, the vertical line can be grounded for only the horizontal blanking period, i.e., about 10 xcexcs. Therefore, the charge overflowing onto the vertical line during the horizontal scanning period still causes the blooming phenomenon.
In the readout mode, when imade signals are sequentially output by horizontal scanning after they are stored in a vertical line, a dummy signal is generated during the storage of the signal in the vertical line. In other words, a smear phenomenon occurs.
In addition, the period for performing the refresh operation in the conventional apparatus is about 10 xcexcs in the horizontal blanking period. The refresh time is short and results in incomplete refreshing and hence an after image phenomenon.
Furthermore, assume that when the conventional photoelectric transducer apparatus is used as a single-plate type solid-state imaging device in a color television video camera, color filters are deposited or adhered onto the pixels. If an alignment scheme such as a Bayer alignment is used to form vertical lines in units of colors, i.e., R, G, and B, at least two vertical lines are required for the pixels of each column. In this case, since the vertical line, portion does not serve as the photosensitive portion, the light-receiving area is reduced by the two vertical lines for each column. In other words, the opening of the aperture is undesirably reduced.
In a conventional photosensitive transducer apparatus, negative and positive voltages are required to bias an output amplifier, and the constitution is thus complicated. It is difficult to read out the signal component without degrading the frequency characteristics.
FIG. 19A is a schematic circuit diagrams of a conventional solid-state image pickup apparatus.
Referring to FIG. 15A, signals from sensors S1 to Sn are respectively amplified by amplifiers A1 and An, and transistors T1 to Tn are sequentially turned on. A dot sequential output appears on an output line 101A. The dot sequential signal is amplified by a buffer amplifier 102A, and the resultant signal appears as an output signal Vout.
In the conventional image pickup apparatus described above, variations in input/output characteristics of the amplifiers A1 to An are included in the sensor signals as the dot sequential output appearing on the output line 101A. As a result, steady pattern noise undesirably occurs.
FIG. 15B shows a schematic arrangement of another conventional photoelectric transducer apparatus.
Referring to FIG. 15B, signals read out from photosensors S1 to Sn are temporarily stored in storage capacitors C1 to Cn. Transistors T1 to Tn are sequentially turned on at timings of a scanning circuit SH, and the readout signals sequentially appear on an output line 101A and are output to an external device through an amplifier 102A.
In the above photoelectric transducer apparatus, however, unnecessary components such as dark signals and drive noise of the photosensors are undesirably included.
Drive noise is defined as noise generated when a photosensor is driven to read out a signal. The drive noise components are noise caused by manufacturing variations such as element shapes and smear caused by element isolation and depending on radiation amounts.
The dark signal is defined as a dark current of a photosensor and greatly depends on accumulation time and temperature of the photosensor.
This drive noise will be described in detail. Variations in drive capacity of a drive element for driving a photoelectric transducer element and variations in capacity of a photoelectric transducer element cause variations in a leakage component of drive pulses. These variation components as an information signal are superposed on a necessary photoelectric transducer signal and are read out. The cause of generation of drive noise will be described below.
FIG. 15C is a schematic view of a photoelectric transducer element described in Japanese Patent Laid-Open Gazette No. 12764/1985. FIG. 15D is a timing chart of drive pulses for driving the photoelectric transducer element shown in FIG. 15C, and FIG. 15E is a chart showing the base potential of the photoelectric transducer element.
Referring to FIG. 15C, the photoelectric transducer element includes a base accumulation type bipolar transistor B, a drive capacitor Cox for reverse- or forward-biasing the transistor B in response to a drive pulse xc3x8r, and a refresh transistor Qr. The transistor B has junction capacitances Cbc and Cbe. It should be noted that Cox, Cbc, and Cbe are referred to as capacitances or capacitors hereinafter, as needed. The capacitances Cox, Cbc, and Cbe are added to obtain a charge storage capacitance Ctot.
The operation of the photoelectric transducer element will be described below.
Assume that the initial value of a base potential VB is given as V0. When the drive pulse xc3x8r is set at a potential Vxc3x8r at time t1, a voltage Va is applied to the base of the transistor B through the drive capacitor Cox. In this case, the voltage Va can be represented as follows:
Va=Cox/(Cox+Cbc+Cbe)xc3x97Vxc3x8r=(Cox/Ctot)xc3x97Vxc3x8rxe2x80x83xe2x80x83(1)
When the drive pulse xc3x8rh is set at a high potential at time t2, a transistor Qr is turned on.
When the transistor B is forward-biased, the base potential VB is abruptly decreased. A time interval TC between time t2 and time t3 is a so-called refresh time interval.
The drive pulse xc3x8r is set at zero at time t3, and a voltage xe2x88x92Va is added to the base voltage VB, so that the base voltage VB is set at V2. This reverse-biased state is the accumulation state.
The above description was confined to one photoelectric transducer element. However, a line or area sensor has a large number of photoelectric transducer elements. The capacitances of the capacitors Cox, Cbc, and Cbe between a large number of photoelectric transducer elements vary by a few fractions of 1%. For example, if the following conditions are given:
Cox=Cbc=Cbe=0.014 pF, and Vxc3x8r=5 V
and the capacitance variation is 0.2%, then a variation xcex94Va in capacitance division voltage Va is about 3 mV.
The variation xcex94Va can be reduced by refreshing. However, when the refresh mode is changed to an accumulation operation mode (time t3), the variation occurs again to produce xcex94Vb. The variation xcex94Vb does not satisfy the relation xcex94Vb=xe2x88x92xcex94Va, and the correlation cannot be established therebetween according to test results.
The above fact is assumed to be derived from different bias voltage dependencies of Cbc and Cbe.
In the next read cycle, when the transistor B is forward-biased, the variation in base potential thereof is approximated as follows:
xcex94V2xcex94Va2+xcex94Vb2+2Kxcex94Vaxcex94bxe2x80x83xe2x80x83(2)
for K equal to xe2x88x921 or more. As a result, the variation xcex94V becomes steady drive noise of about 4 to 5 mV.
The variation in leakage component of such a drive pulse (to be referred to as drive noise hereinafter) is eliminated according to the following conventional technique. That is, the above drive noise is stored in a memory means and is read out and subtracted from the signal read out from the sensor to obtain a true information signal.
The conventional drive noise correction technique described above causes a bulky, expensive photoelectric transducer element which does not have any industrial advantage.
In particular, in case the numbers of elements arranged in the horizontal direction and vertical direction are five hundred respectively, an area sensor requires 250,000 photoelectric elements arranged in a matrix form. In addition, when the resolution of the sensor is also taken into consideration, a memory of several megabits is required.
The unnecessary signals such as drive noise and a dark signal pose serious problems when an image of a dark object is to be picked up, i.e., image pickup at a low intensity. In the low-intensity image pickup mode, an information signal level is low and accordingly the S/N ratio is degraded. As a result, image quality is degraded. In order to improve image quality, the unnecessary signals must be reduced.
As described above, however, the dark signal primarily depends on temperature and charge accumulation time, although the drive noise rarely depends thereon. If these unnecessary signals are to be eliminated, the dark signal must be separated from the drive noise and a correction coefficient must be determined, thus requiring a large-capacity memory. As a result, signal processing is complicated and expensive, and an image pickup apparatus is undesirably bulky.
It is an object of the present invention to provide a photoelectric transducer apparatus capable of solving the conventional drawbacks described above.
It is another object of the present invention to provide a simple photoelectric transducer apparatus capable of eliminating variations in dark voltage.
It is still another object of the present invention to provide a photoelectric transducer apparatus comprising optical information storing means for storing optical information (light or bright signals) read out from a photoelectric transducer element and dark voltage storing means for storing a voltage corresponding to a dark voltage component read out from the photoelectric transducer element, wherein actual optical information stored in the optical information storing means is simultaneously read out together with the dark voltage component stored in the dark voltage storing means onto separate output lines, thereby correcting the information corresponding to the dark voltage in units of optical sensor cells and hence removing noise caused by variations in dark voltage from the output signal from the photosensor cells.
In order to achieve the above object, according to an aspect of the present invention, there is provided a photoelectric transducer apparatus having a plurality of photoelectric transducer elements each having a capacitor electrode formed on a control electrode of a corresponding semiconductor transistor, the apparatus being adapted to sequentially select each element in units of lines, to control a potential of the control electrode of the selected photoelectric transducer element through the capacitor electrode, to store carriers in the control electrode region, and to read out a signal component corresponding to the amount of charge stored in the control electrode region, comprising: optical information storing means for storing optical information read out from the photoelectric transducer element; and dark voltage storing means for storing a voltage corresponding to a dark voltage read out from the photoelectric transducer element, wherein actual optical information stored in the optical information storing means and information corresponding to the dark voltage component stored in the dark voltage storing means are simultaneously read out onto different information output lines.
The information corresponding to the dark voltage component stored in the dark voltage storing means is read out onto the information output line therefor, and at the same time the information corresponding to the dark voltage is corrected in units of photosensor cells, thereby eliminating noise caused by variations in dark voltage.
The noise corresponding to the dark voltage component can, therefore, be processed within the sensor. An external circuit or the like need not be used to easily constitute a system configuration, thereby obtaining a low-cost photoelectric transducer apparatus.
It is still another object of the present invention to provide an imaging element and an apparatus using the same, wherein the after image, blooming, and smearing can be prevented with a simple construction.
It is still another object of the present invention to provide a color imaging element having a large aperture.
In order to achieve these objects, according to another aspect of the present invention, there is provided a photoelectric transducer apparatus comprising:
a plurality of photoelectric transducer cells;
a signal read line for reading out signals from the plurality of photoelectric transducer elements; and
a plurality of capacitors for selectively storing the signals read out through the signal read line.
According to this aspect of the present invention, since the plurality of capacitors for selectively storing the signals read out through the signal read line are provided, the image signal appearing on the vertical line can be shortened, thereby reducing the frequency of occurrence of the blooming and smearing phenomena. Since the capacitor can be disconnected from the pixel after the image signal is stored in the capacitor, the refresh time can be prolonged to reduce the occurrence of the after image phenomenon. In addition, if the photoelectric transducer apparatus is used in a color video camera, the number of capacitors can be that of the color signals of the row pixels, and only one vertical line is used, thereby increasing the aperture.
It is still another object of the present invention to provide a photoelectric transducer apparatus wherein a single power source can be used without degrading the signal component of the read signal.
In order to achieve the above object, according to still another aspect of the present invention, there is provided a photoelectric transducer apparatus for reading output a read signal from a photoelectric transducer element through an amplifier after the read signal is temporarily stored in a storing means, comprising switching means for properly applying a bias voltage to the storing means.
With the above arrangement, the reference potential of the store capacitor can be properly changed to use a single power source without degrading the signal component of the read signal.
It is still another object of the present invention to provide a photoelectric transducer apparatus little subjected to smearing or blooming.
In order to achieve the above object, according to still another aspect of the present invention, a capacitor is arranged in a vertical signal line through a switch to store the signal from the photoelectric transducer cell in the capacitor, thereby resetting the vertical signal line, so that the signal component in the capacitor is free from smearing or blooming.
It is another object of the present invention to eliminate variations in drive noise in units of sensor cells.
It is still another object of the present invention to compensate for variations in electrical characteristics of a plurality of amplifiers arrangement for sensor cells.
In order to achieve the above objects according to an aspect of the present invention, there is provided a solid state image pickup apparatus having a selector for selecting a plurality of sensor signals through corresponding amplifiers, comprising a processing circuit for calculating a difference between a selected sensor signal and a reference signal selected through the same circuit for selecting the sensor signal.
The sensor signal selected by the selector, therefore, includes a noise component caused by variations in amplifier characteristics since the sensor signal is amplified by the corresponding amplifier. For this reason, the reference signal is selected through the same amplifier which has amplified the sensor signal, so that the amplifier noise is superposed on the reference signal. A difference between the selected sensor signal and the selected reference signal is calculated to eliminate the noise component.
According to another aspect of the present invention, there is provided a photoelectric transducer apparatus having storage means for storing a signal from a photoelectric transducer element, wherein the storage means comprises first storage means for storing the signal read out from, the photoelectric transducer element and second storage means for storing a residual signal left after the photoelectric transducer element is refreshed, and further comprising difference processing means for calculating a difference between the readout and residual signals respectively stored in the first and second storage means.
Since the residual signal obtained upon completion of refreshing is subtracted from the readout signal, the unnecessary components such as a dark signal and drive noise of the photoelectric transducer element can be eliminated.
A MOS, electrostatic induction, or base accumulation type photosensor may be used as a photoelectric transducer element.
xe2x80x9cRefreshingxe2x80x9d of the photoelectric transducer element means erasure of optical information of the photoelectric transducer element. In some photosensors, optical information is erased simultaneous when the information is read out. However, in some photosensors, optical information is kept unerased even after the information is read out.
According to still another aspect of the present invention, in order to eliminate the conventional drawbacks described above, there is provides a solid state image pickup apparatus comprising a plurality of photoelectric transducer elements, first storage means, arranged in units of photoelectric transducer elements, for storing a video signal, second storage means, arranged in units of photoelectric transducer elements, for storing noise components, first readout means for simultaneously and independently reading out signals for photoelectric transducer elements of a plurality of horizontal lines from the first storage means, and second readout means for adding signals for the photoelectric transducer elements of the plurality of horizontal lines from the second storage means and for reading out a sum signal.
With the above arrangement, it is assumed that the drive noise is generated as a sum of noise components generated in the refresh, charge accumulation, and readout modes of the photoelectric transducer element and the drive noise level is substantially identical in each mode. A difference between the photoelectric transducer signal read out upon completion of exposure and drive noise read out in the photoelectric transducer signal readout mode is calculated to eliminate the drive noise. It should be noted that the noise components are read out after they are added, thereby reducing the number of read lines.
According to still another aspect of the present invention, in order to eliminate the conventional drawbacks described above, there is provided a solid state image pickup apparatus comprising photoelectric transducer elements, a plurality of storage capacitors for storing readout signals when the photoelectric transducer elements are read-accessed a plurality of times, dot sequential processing means for converting signals from the storage capacitors into a dot sequential signal, and clamping means for clamping some components of the dot sequential signal from the dot sequential processing means.
With the above arrangement, it is assumed that the drive noise is generated as a sum of noise components generated in the refresh, charge accumulation, and readout modes of the photoelectric transducer element and the drive noise level is substantially identical in each mode. The photoelectric transducer signal read out upon completion of exposure and drive noise read out in the photoelectric transducer signal readout mode are converted into a dot sequential signal, and the drive noise component is clamped, thereby eliminating the drive noise included in the photoelectric transducer signal components.
The above and other objects, features, and advantages of the present invention will be apparent from the following detailed description in conjunction with the accompanying drawings.